Based on what we know of our universe, many cosmologists believe it all began when the Big Bang spat a bunch of matter into being. But a lot of questions remain. That's why a Canadian astrophysicist has suggested that there's another possibility. Maybe our universe began with a 4D black hole?

Writing in Nature, Zeeya Merali explains the theory suggested by astrophysicist Niayesh Afshordi, at the Perimeter Institute for Theoretical Physics:

The standard Big Bang model tells us that the Universe exploded out of an infinitely dense point, or singularity. But nobody knows what would have triggered this outburst: the known laws of physics cannot tell us what happened at that moment. . . .

Afshordi and his colleagues [suggest that] our three-dimensional (3D) Universe is a membrane, or brane, that floats through a ‘bulk universe’ that has four spatial dimensions.

Ashfordi's team realized that if the bulk universe contained its own four-dimensional (4D) stars, some of them could collapse, forming 4D black holes in the same way that massive stars in our Universe do: they explode as supernovae, violently ejecting their outer layers, while their inner layers collapse into a black hole.

In our Universe, a black hole is bounded by a spherical surface called an event horizon. Whereas in ordinary three-dimensional space it takes a two-dimensional object (a surface) to create a boundary inside a black hole, in the bulk universe the event horizon of a 4D black hole would be a 3D object — a shape called a hypersphere. When Afshordi’s team modelled the death of a 4D star, they found that the ejected material would form a 3D brane surrounding that 3D event horizon, and slowly expand.

The authors postulate that the 3D Universe we live in might be just such a brane — and that we detect the brane’s growth as cosmic expansion. “Astronomers measured that expansion and extrapolated back that the Universe must have begun with a Big Bang — but that is just a mirage,” says Afshordi.

Afshordi and his colleagues published their theory in a paper on ArXiv last week, suggesting that it could help explain the uniform temperature of the universe (which is still a mystery). It could also provide a reason for the Big Bang in the first place. What triggered that hypothetical singularity to become a massive, ever-expanding explosion?

“For all physicists know, dragons could have come flying out of the singularity,” Afshordi told Nature. At least his theory has the virture of explaining that initial explosion, and the aftermath — even if it doesn't have dragons.